6l2 



BETULACEAE. 



VOL. I. 



12. Betula nana L. Dwarf or Alpine Birch. 



Fig- 1505- 



Betula nana L. Sp. PI. 983. 1753. 

 B. Michauxii Spach, Ann. Sci. Nat. (II.) 15: 195. 1841. 



A low diffuse shrub, similar to the preceding 

 species, but the twigs glandless, puberulent or 

 glabrous. Leaves orbicular, obovate, or reniform 

 and wider than long, bright green, firm, glabrous, 

 on both sides when mature, deeply and incisely 

 crenulate, rounded at the apex, rounded, obtuse 

 or cuneate at the base, 3"-io" long; petioles rarely 

 more than i" long; staminate aments i'-i' long, 

 solitary or clustered; pistillate aments oblong, sessile 

 or short-peduncled, erect or somewhat spreading, 

 3"-5" long; fruiting bracts glabrous, the lower usu- 

 ally 3-lobed, the upper ovate or lanceolate, mostly 

 entire ; nut oblong, wingless or narrowly winged. 



Greenland and Labrador to Hudson Bay. Also in 

 northern Europe and Asia. May-June. 



5. ALNUS (Tourn.) Hill, Brit. Herb. 510. 1756. 



Shrubs or trees, with dentate or serrulate leaves, few-scaled or naked buds, and flowers 

 of both kinds in aments, expanding before, with or after the leaves, making in most species 

 their first appearance during the preceding season, the staminate pendulous, the pistillate 

 erect, clustered. Staminate flowers 3 or sometimes 6 in the axil of each bract, consisting of a 

 mostly 4-parted perianth, 4 stamens and subtended by I or 2 bractlets; filaments short, simple; 

 anther-sacs adnate. Pistillate flowers 2-3 in the axil of each bract, without a perianth, but 

 subtended by 2-4 minute bractlets ; ovary sessile, 2-celled ; styles 2 ; bracts woody, persistent, 

 5-toothed or erose. Nut small, compressed, wingless or winged. [Ancient Latin name de- 

 rived from the Celtic, in allusion to the growth of these trees along streams.] 



About 14 species, natives of the northern hemisphere and the Andes of South America. Besides 

 the following, some 4 others occur in western North America. Type species : Alnus vulgaris Hill. 

 Nut bordered by a membranous wing on each side. i. A. Alnobctula. 



Nut acute-margined, wingless. 



Leaves obovate, broadly oval or suborbicular, dull ; aments expanding long before the leaves. 

 Leaves finely pale-tomentose or glaucous beneath. 2. A. incana. 



Leaves green and glabrous or pubescent beneath, obovate to suborbicular. 



Leaves finely serrulate ; native. 3. A. rugosa. 



Leaves dentate-serrate; twigs glutinous; introduced tree. 4. A. Alnus. 



Leaves mostly oblong, bright green and shining above ; aments expanding in late summer or 

 autumn. 5. A. maritima. 



i. Alnus Alnobetula (Ehrh.) K. Koch. Green or Mountain Alder. Fig 1506. 



Betula Alnobetula Ehrh. Beitr. 2: 72. 1788. 



Alnus viridis DC. Fl. Fr. 3 : 304. 1805. 



A. crispa Pursh, Fl. Am. Sept. 623. 1814. 



A. Mitchelliana M. A. Curtis ; A. Gray, Am. Journ. Sci. 



42 : 42. 1842. 



Alnus Alnobetula K. Koch, Dendr. 2 1 : 625. 1872. 

 Alnus mollis Fernald, Rhodora 6: 162. 1904. 



A shrub, 2-io high, the young leaves glutinous 

 and more or less pubescent, the twigs glabrous or 

 pubescent. Leaves oval or ovate, obtuse or acute, 

 sharply and more or less irregularly serrulate or 

 incised-serrulate, when mature dark green and gla- 

 brous above, light green and glabrous or pubescent 

 beneath, 2'-$' long, I '-3' wide; petioles 4"-! 2" long; 

 aments expanding with the leaves, the staminate 

 slender, naked, I'-2$' long, the pistillate oblong or 

 ovoid-oblong, slender-peduncled, becoming 4"-io" 

 long and 4"-5" in diameter in fruit, their bracts ir- 

 regularly 5-toothed ; nut oblong, the thin wings about 

 as broad as the body. 



Newfoundland to Manitoba, Massachusetts, New York, 

 Michigan, and in the higher Alleghanies to North Caro- 

 lina. Also in Europe and Asia. June. Consists of many 

 races, differing mainly in the amount of pubescence. 



